Self-assembled mid-infrared metasurfaces for high-contrast femtosecond switching

The ultrafast and large-contrast switching of light is essential to the realization of high-speed, energy-efficient optical logic elements and sources. Such functionality requires optical materials or systems whose optical response can be rapidly and dramatically tuned under an external stimulus. However, achieving high-contrast switching, in sub-picosecond time scales, with low power over macroscopic length scales, is exceedingly difficult. We utilize self-assembled, cavity-coupled tin-doped indium oxide nanocrystal monolayers to achieve sub-picosecond, high-contrast switching across the mid-infrared (from 2.5 to 6.1 μm). We achieve reflection modulation up to 40% absolute and 1200% relative depth, under moderate fluences of a few hundred μJ/cm2, whereas our pump-probe experiments reveal an upper bound on response times of 210 femtoseconds. The strong and ultrafast nonlinearity, along with the broad spectral tunability and scalable fabrication method, make the demonstrated system an appealing platform for a range of mid-infrared applications, including ultrafast optical switching and mode-locked lasers.